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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 107 (2021), No. 11

  • Fault Detection Technique for Hierarchical Monitoring of Steel Making Facilities Based on Data Science

    pp. 897-905

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    DOI:10.2355/tetsutohagane.TETSU-2021-031

    In steel making processes, influence of an equipment fault on production operation is significant. It is strongly required to detect an equipment fault at an early stage and to prevent the damage. Therefore, fault detection technique for steel making facilities based on data science is developed as an online-monitoring system. One of main features of the developed system is hierarchical monitoring consisting of three levels such as an entire process, facilities and sensors. Another is display of heat-mapping according to the degree of anomaly for huge number of monitoring items. Some anomaly signs at the hot rolling process where the system has been developed are successfully detected.
  • Effect of Humidity of Air on Hydrogen Absorption into Fe with Rust Layer Containing MgCl2 during Atmospheric Corrosion

    pp. 906-914

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    DOI:10.2355/tetsutohagane.TETSU-2021-081

    An Fe plate, whose one side was electro-polished and the other was covered with the rust layer containing 25.7 g·m−2 MgCl2, was used as the specimen to investigate the effect of humidity on the hydrogen absorption of the plate during atmospheric corrosion. The specimen was subjected to an electrochemical hydrogen-absorption test during which the rusted surface was exposed to the air with controlled relative humidity (RH) and atmospheric corrosion occurred on it. When the rusted surface was subjected to dry (RH 0%)–wet (RH 27%) cycle tests for 10.8 ks each, the anodic current density corresponding to the hydrogen-absorption rate was measured on the hydrogen detection surface. The maximum current density was almost independent of the cycle during the first 10 cycles, after which it decreased with an increase in the cycle, reaching almost a steady value after about 40 cycles. After 55 cycles of the dry–wet cycle test, the specimen was subjected to an electrochemical hydrogen-absorption test to obtain the relationship between the steady-state hydrogen-absorption rate and RH. Hydrogen absorption was observed at RH over about 15%, and the absorption rate increased rapidly with an increase in RH, reached a maximum at RH of about 30%, and then decreased rapidly. When RH increased beyond 40%, the absorption rate increased again, reached a maximum value at RH of 80%, and then decreased gradually. The specimen with the rust layer containing 39.8 g·m−2 MgCl2 also showed two peaks in the hydrogen-absorption rate versus RH plot.
  • Hydrogen Absorption into Fe Plates with Rust Layers Containing Various MgCl2 Amounts during Atmospheric Corrosion with Controlled Humidity

    pp. 915-922

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    DOI:10.2355/tetsutohagane.TETSU-2021-082

    Fe plates with rust layers containing various MgCl2 amounts were prepared as specimens. Each specimen was subjected to dry−wet repeated cycle test beyond 50 cycles, and then subjected to electrochemical hydrogen absorption test under atmospheric corrosion in the air with controlled relative humidity (RH). For an MgCl2 amount of 39.8 g·m−2, a hydrogen absorption rate (iH) started to increase from an RH around 15%, steeply increased with an increase in RH up to about 30%, steeply decreased up to about 35%, gradually increased up to 65% and gradually decreased up to about 92%. A decrease in MgCl2 amount in the range between 0.514 and 39.8 g·m−2 induced a decrease in iH in the whole RH range. The maximum iH at an RH around 30% increased with an increase in MgCl2 amount in the rust layer. Besides, the RH where the maximum iH was obtained beyond an RH of 40% increased with a decrease in MgCl2 amount. From theoretical relationship between RH and thickness of MgCl2 solution film on the Fe plate without rust layer, it is found that the solution film thicknesses at the RHs were about 0.18 mm, almost independent of MgCl2 amount. In addition, thicknesses of the rust layers containing 25.7 and 39.8 g·m−2 MgCl2 were measured to be almost 0.18 mm each other. The trends of iH depending on MgCl2 amount were tried to be explained using nature of deliquescence for MgCl2.
  • Electron Theory Calculation of Thermodynamic Properties of Steels and Its Application to Theoretical Phase Diagram of the Fe-Mo-B Ternary System

    pp. 923-933

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    DOI:10.2355/tetsutohagane.TETSU-2021-078

    In this study, the ground structures of the Fe-Mo-B ternary systems were estimated by first-principles calculations based on genetic algorithm and the free energies of their structures are evaluated by electronic calculations and statistical thermodynamic techniques. In addition, the phase diagram at finite temperature was theoretically constructed using the calculated free energies and the result was compared with the experimental knowledge. The space groups and compositions of many ground structures obtained by the calculations correspond well with the experimental findings, but the agreement is not perfect. However, by including metastable structures by only a few kJ/mol than the ground state, it becomes clear that the appearance of almost all structures can be predicted based on this technique. The new calculation technique of such theoretical phase diagrams suggested in the present study is expected to open up the possibility of estimation of unknown phase diagram, reexamination of experimental phase diagrams and discovery of new phases. On the other hand, examining the calculation conditions for improving the accuracy of energy calculation, consideration of the anharmonicity of atomic vibration, magnetic entropy effect, handling of solid solution, etc. are mentioned as problems requiring further consideration.
  • Development of Auto-Searching Method of Brittle Fracture Initiation Point Based on River-Pattern and Tear Ridge

    pp. 934-943

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    DOI:10.2355/tetsutohagane.TETSU-2021-025

    A method to automatically search brittle fracture initiation point is proposed. The method calculates flow paths from each edge of an image to opposite side along flow potential. The potential is derived from the direction of river-pattern and tear ridge. The most concentrated position of flow paths is determined as a fracture initiation point. The method achieves 99% precision and high speed analysis for low magnification images of 206 thick plate samples. Furthermore, discriminating ductile fracture by machine learning and excluding noise potential obtained from it, and considering local feature of river-pattern that spread radially near a fracture initiation point, the method precision is improved for high magnification images. Using fractal feature of brittle fracture and repeatedly applying the method, a fracture initiation point is determined within the size of a single grain level.
  • Application of Molecular Dynamics Calculations to Elucidation of the Mechanism of Hydrogen-Induced Crack Initiation in Fracture Toughness Tests Using Tempered Martensitic Steels

    pp. 944-954

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    DOI:10.2355/tetsutohagane.TETSU-0201-043

    It is well known that the presence of hydrogen deteriorates mechanical properties of steels, that appears as reduced fracture toughness, shorter fatigue lifetime, etc.; these phenomena are recognized as hydrogen embrittlement. The effect of hydrogen on crack initiation of fracture toughness test has been investigated using a combination of experimental and computational approaches. Tempered lath martensitic steel was subjected to fracture toughness test with monotonically rising load in air and high-pressure hydrogen gas. While crack propagated continuously within grains in air, cracking in hydrogen grew by linking isolated interface failure ahead of a main crack tip. Then, to understand the nucleation mechanism of isolated failure in the presence of hydrogen, the tensile simulations of twist grain boundaries (TGBs) rotated along <110> axis at various angles were conducted using molecular dynamics calculations. While the dislocation emission from TGB rotated 70° is dominant deformation mode in the absence of hydrogen, the rupture along TGB rotated 110° and 170° without stress relaxation due to dislocation emission is dominant deformation mode in the presence of hydrogen. As a consequence, it is indicated that the origin of hydrogen-induced isolated crack initiation in the vicinity of fatigue pre-crack is the rupture along the block boundaries within martensitic structure due to hydrogen-induced inhibition of dislocation emission from GBs.
  • Internal and External Hydrogen-related Loss of Ductility in a Ni-based Superalloy 718 and Its Temperature Dependence

    pp. 955-967

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    DOI:10.2355/tetsutohagane.TETSU-2021-056

    Toward a better understanding of the hydrogen embrittlement characteristics in nickel-based superalloy 718, tensile tests were performed under hydrogen pre-charged states (internal hydrogen) as well as in hydrogen gas environment (external hydrogen) at various temperatures ranging from −196 to 300°C. Under the internal hydrogen conditions, hydrogen-induced loss of ductility was maximized at around 25°C, while it was recovered with increasing/decreasing test temperature and almost fully mitigated particularly at −196°C. On the other hand, under the external hydrogen conditions, deleterious impact of hydrogen on the ductility monotonically increased with temperature elevation. Scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) analyses on post-mortem samples revealed that the microstructural initiation sites of hydrogen-induced micro-cracks in internal hydrogen states were annealing twin boundaries or crystallographic slip planes (i.e., {111} planes) at −40~300°C wherein the loss of ductility was substantial, albeit intergranular fracture prevailed at −196°C, accompanying minimum embrittlement effect. Meanwhile, in the case of external hydrogen states, the fracture modes were transitioned from intergranular to slip plane cracking with increasing temperature in response to the augmentation of embrittlement magnitude. The rationales of these multiple hydrogen-related failure modes and their roles on macroscale material performance are discussed on the basis of hitherto-known, unique deformation mechanisms driving the plasticity in this alloy in addition to the hydrogen diffusion rate/pathways which are strongly dependent on temperature.
  • Effect of Deformation Temperature on Mechanical Properties in 1-GPa-grade TRIP Steels with Different Retained Austenite Morphologies

    pp. 968-976

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    DOI:10.2355/tetsutohagane.TETSU-2021-058

    The effect of deformation temperature on the mechanical properties of 1-GPa-grade TRIP steels with different retained austenite (γR) morphologies was studied. The temperature dependence on the deformation-induced martensitic transformation behavior was also investigated. The uniform elongation below room temperature was relatively large in the needle-like γR steel, whereas the tensile strength at each temperature was almost the same as it was independent of the γR morphology. The better tensile strength–uniform elongation balance was obtained at 373 K for the needle-like γR steel and at 473 K for blocky γR one. The mechanical stability of γR was higher in the needle-like γR steel, according to the x-ray diffraction experiments. γR was mechanically stable with increasing temperature, but its volume fraction decreased at temperatures above 473 K because of the deformation-induced bainitic transformation. In this paper, the quantitative conditions of deformation-induced transformation to obtain better uniform elongation in the 1-GPa-grade TRIP steels are summarized from the viewpoints of the volume fraction of deformation-induced martensite and the transformation rate.
  • Evaluation of Cleavage Fracture Behavior of C14 Fe2W Laves Phase by First Principle Calculation and Crystal Orientation Analysis

    pp. 977-985

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    DOI:10.2355/tetsutohagane.TETSU-2021-069

    Cleavage fracture of C14 Fe2W Laves phase was investigated using crystal orientation measurement with scanning electron microscopy and first-principles calculations. Trace analysis of the orientations of cleavage planes reveals that cleavage fracture occurred at five types of crystal planes of (0001), {1100}, {1120}, {1101} and {1122}, among which the fracture at (0001) is the most preferable. The first-principle calculations of the surface energy for fracture, Young's modulus, and Poisson's ratio showed that the minimum fracture toughness value of 1.62 MPa·m1/2 was obtained at (0001). The tendency that the values of calculated fracture toughness become larger with the higher indexed planes is almost the same as the frequency of the types of cleavage planes in the trace analysis. It is concluded that the fracture toughness of C14 Fe2W Laves phase is controlled by the surface energy for fracture and Young’s modulus.
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  • Effect of Strain Rate on Hydrogen Embrittlement Susceptibility of Tempered Martensitic Steel and the Rate-Determining Process

    pp. 986-995

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    DOI:10.2355/tetsutohagane.TETSU-2021-057

    The effect of strain rate on hydrogen embrittlement susceptibility of high strength tempered martensitic steel was investigated by tensile tests under cathodic hydrogen charge. Fracture elongation was decreased with a decrease in strain rate and increase in diffusible hydrogen concentration. The mechanism of hydrogen embrittlement susceptibility was investigated based on vacancy-type lattice defects formation by positron lifetime spectroscopy. The clear correlation was not comfirmed between strain rate and the parameters of average positron lifetime, dislocation density and vacancy density. These parameters decreased with a decrease in fracture elongation, it means that these parameters did not reflect hydrogen embrittlement susceptibility. On the other hand, vacancy clustering was promoted with a decrease in strain rate. Therefore, it is assumed that strain rate dependence of hydrogen embrittlement susceptibility is determined by vacancy clustering process.
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